Which syndrome cannot be definitively diagnosed in children younger than three years old?

Alcohol-Related Neurodevelopmental Disorder (ARND) cannot be definitively diagnosed in children younger than three years old primarily due to the challenges associated with assessing neurodevelopmental impairments in very young children. At this age, developmental milestones can vary widely, and identifying specific deficits or disorders can be particularly difficult.

In contrast, Fetal Alcohol Syndrome (FAS) is characterized by a distinct set of physical, behavioral, and cognitive features that can sometimes be identified in younger children, particularly the presence of growth deficiencies and certain facial dysmorphologies. Alcohol-Related Birth Defects may also be diagnosed in infancy based on physical structural anomalies. Fetal Alcohol Spectrum Disorder (FASD) serves as an umbrella term that encompasses conditions related to alcohol exposure during pregnancy, but does include specific criteria that allow for broader diagnosis.

Thus, the definitive diagnosis of ARND relies on a clearer understanding of cognitive and behavioral abilities that may not be fully apparent until the child is older or has reached a more developed stage of cognitive function and learning ability.